Piercer plug

ABSTRACT

A piercer plug with increased wear resistance is provided. A piercer plug includes: a plug body; and a sprayed coating formed on the surface of the plug body. The sprayed coating contains an iron-based alloy and oxides thereof. The chromium concentration determined by analyzing the sprayed coating with X-ray fluorescence analysis is 3 to 20 mass %.

TECHNICAL FIELD

The present invention relates to a piercer plug.

BACKGROUND ART

Conventionally, a piercer plug for piercing and rolling to produce a seamless steel pipe is provided with a scale coating on its surface before use to ensure the thermal insulation, lubrication and seizure resistance of the surface.

A scale coating gradually wears down during each round of piercing/rolling. When the scale coating wears out and base material (i.e., plug body) is exposed, the base material may be eroded and/or seize upon the material to be processed. A scale coating significantly wears down during piercing of a material that is difficult to process, such as stainless steel; the coating may wear out just after several passes. Each time this occurs, heat treatment is necessary to form a scale coating once again, which requires several hours to several dozens of hours, meaning poor efficiency.

WO 2009/057471 proposes forming a sprayed coating made from iron and oxides on the surface of the base material of the piercer plug. WO 2014/034376 discloses a piercer plug including a sprayed coating containing iron and iron oxides and, in addition, in mass %, 0.015 to 0.6% C, 0.05 to 0.5% Si, 0.1 to 1.0% Mn, and 0 to 0.3% Cu.

DISCLOSURE OF THE INVENTION

Sprayed coatings have better adhesion with respect to base material and better wear resistance than scale coatings, and can be formed in several minutes to several dozens of minutes. As such, sprayed coatings have longer lives than scale coatings, and, even when a sprayed coating has worn out, it can be restored in a short period of time. Meanwhile, it is desirable to further increase the life of piercer plugs to increase the manufacture efficiency of seamless steel pipes. It is thus desirable to further increase the wear resistance of coatings.

An object of the present invention is to provide a piercer plug with further increased wear resistance.

A piercer plug according to an embodiment of the present invention includes: a plug body; and a sprayed coating formed on a surface of the plug body. The sprayed coating contains an iron-based alloy and an oxide of the iron-based alloy. A chromium concentration determined by analyzing the sprayed coating with X-ray fluorescence analysis is 3 to 20 mass %.

The present invention provides a piercer plug with further increased wear resistance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal cross-sectional view of a piercer plug according to an embodiment of the present invention.

FIG. 2 illustrates exemplary equipment used to form a sprayed coating.

FIG. 3 is a cross-sectional view of a cored wire.

FIG. 4 is a longitudinal cross-sectional view of a piercer plug according to another embodiment of the present invention.

FIG. 5 is a longitudinal cross-sectional view of a piercer plug according to yet another embodiment of the present invention.

FIG. 6 is a cross-sectional microphotograph of a sprayed coating containing no Cr.

FIG. 7 is a cross-sectional microphotograph of a sprayed coating containing Cr.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will now be described in detail with reference to the drawings. The same or corresponding parts in drawings are labeled with the same characters and their description will not be repeated. The size ratios between components shown in drawings do not necessarily indicate the actual size ratios.

[Construction of Piercer Plug]

FIG. 1 is a longitudinal cross-sectional view of a piercer plug 10 according to an embodiment of the present invention. The piercer plug 10 includes a plug body 11 and a sprayed coating 12.

The plug body 11 is projectile-shaped. Specifically, the plug body 11 has a circular transverse section and is shaped such that its outer diameter increases as it goes from the tip of the plug body 11 toward the rear body's end. The plug body 11 is formed from an iron-based alloy, for example.

The sprayed coating 12 is provided on the surface of the plug body 11. The sprayed coating 12 covers the entire surface of the plug body 11 except for the rear end face of the plug body 11. The thickness of the sprayed coating 12 may not be constant. The sprayed coating 12 is preferably formed such that its portions overlying the tip 11 a of the plug body 11 are thicker than those overlying the body's trunk portion 11 b.

The sprayed coating 12 contains at least an iron-based alloy and oxides thereof. The sprayed coating 12 may contain other compounds.

The iron-based alloy in the sprayed coating 12 is mainly composed of iron (Fe), and also contains carbon (C), silicon (Si), manganese (Mn) and chromium (Cr), for example. The iron-based alloy in the sprayed coating 12 may contain only one or some of C, Si, Mn and Cr, and may contain elements other than C, Si, Mn and Cr. When viewed microscopically, the chemical composition of the iron-based alloy in the sprayed coating 12 may not be uniform. For example, when viewed microscopically, portions containing almost no Cr and portions with high Cr contents may be present in a mixed manner.

The oxides in the sprayed coating 12 are the oxides resulting from the iron-based alloy being oxidized. Specifically, the oxides in the sprayed coating 12 are iron oxides and complex oxides of iron and chromium, for example. The iron oxides may be FeO or Fe₃O₄, for example. The complex oxides of iron and chromium may be (Fe,Cr)₃O₄, for example. The oxides in the sprayed coating 12 may contain oxides of other metals.

The higher the proportion of metal (i.e., iron-based alloy) that is an ingredient in the sprayed coating 12, the better the adhesion with respect to the plug body 11. On the other hand, the higher the proportion of oxides, the better the heat insulation. Although not limiting, the proportion of oxides in the sprayed coating 12 is preferably 25 to 80 vol %, and more preferably 35 to 65 vol %. Further, it is preferable that portions of the coating near the plug body 11 have high proportions of metal and portions located toward the surface have higher proportions of oxides. Such a construction will further increase the adhesion with respect to the plug body 11. The volume ratio of oxides can be determined by observing a cross section of the sprayed coating 12 and doing calculations.

In the piercer plug 10 according to the present embodiment, the chromium concentration determined by analyzing the sprayed coating 12 with X-ray fluorescence analysis (hereinafter referred to as “XRF-Cr concentration”) is 3 to 20 mass %.

An XRF-Cr concentration of 3 mass % or higher provides a better wear resistance than a concentration below 3 mass %. This is presumably because the complex oxides of iron and chromium increase the hardness of the sprayed coating 12. On the other hand, an XRF-Cr concentration exceeding 20 mass % results in insufficient lubrication of the sprayed coating 12, which reduces piercing efficiency. The lower limit for XRF-Cr concentration is preferably 5 mass %, and more preferably 8 mass %. The upper limit for XRF-Cr concentration is preferably 18 mass %, and more preferably 16 mass %.

XRF-Cr concentration can be measured in the following manner: an X-ray is directed into the sprayed coating 12 through its surface, and the resulting X-ray fluorescence is detected with a detector. The incident X-ray is generated by a 3 mm ø spot collimator with a target of Rh and an output of 40 kV×100 μA. The detector is a Si drift detector. The concentration of Cr is determined as a mass percentage, where the denominator is the total amount of all the elements detected. The numerator for XRF-Cr concentration includes both the Cr amount in the iron-based alloy and the Cr amount in the oxides.

Preferably, in the piercer plug 10 according to the present embodiment, the iron concentration determined by analyzing the sprayed coating 12 with X-ray fluorescence analysis is 50 mass % or higher. The determination of iron concentration by analyzing the coating with X-ray fluorescence analysis involves the same measurement method as for XRF-Cr concentration.

[Method for Manufacturing Piercer Plug]

An exemplary method for manufacturing the piercer plug 10 will be described below. The method described below is exemplary only, and a method for manufacturing the piercer plug 10 is not limited thereto.

A plug body 11 is prepared. The plug body 11 may be any plug body known to a person skilled in the art.

A sprayed coating 12 is formed on the plug body 11. The sprayed coating 12 may be formed using an arc sprayer 20, shown in FIG. 2.

The arc sprayer 20 includes a spraying gun 21 and a rotating base 24. The spraying gun 21 generates an arc at the tips of a positive-electrode wire 22 and a negative-electrode wire 23 to melt metal, which is then ejected by means of compressed air.

The chemical composition of, and the XRF-Cr concentration in, the sprayed coating 12 may be regulated by adjusting the chemical compositions of the positive- and negative-electrode wires 22 and 23. The positive- and negative electrode wires 22 and 23 may have the same chemical composition or may have different chemical compositions. If wires of different chemical compositions are used, metal from the positive-electrode wire 22 and metal from the negative-electrode wire 23 mix together to form a pseudo-alloy.

Although not limiting, the positive- and negative-electrode wires 22 and 23 may be made of carbon steel or stainless steel, for example. Alternatively, the positive- and negative-electrode wires 22 and 23 may be made of cored wire 30, shown in FIG. 3. The cored wire 30 includes an outer sheath 31 made from carbon steel and a filler 32 filling the outer sheath 31. The chemical composition of the metal ejected from the spraying gun 21 may be changed at will by changing the type of the filler 32.

The longer the distance between the tip of the spraying gun 21 and the surface of the plug body 11 (hereinafter referred to as “spraying distance”), the higher the proportion of oxides in the sprayed coating 12. This is because the oxidation of metal ejected from the tip of the spraying gun 21 progresses along the spraying distance. Although not limiting, the spraying distance may be 100 to 1400 mm, for example. Further, spraying at gradually increasing spraying distances results in higher proportions of metal in portions near the plug body 11 and higher proportions of oxides in portions located toward the surface.

As discussed above, the numerator for XRF-Cr concentration includes both the Cr amount in the iron-based alloy and the Cr amount in the oxides. As such, the XRF-Cr concentration does not significantly change even if the proportion of oxides in the sprayed coating 12 changes. Thus, the XRF-Cr concentration does not significantly change even when the spraying distance is changed.

Spraying is performed while the rotating base 24 rotates the plug body 11 about its axis until the thickness of the sprayed coating 12 reaches a predetermined level. Although not limiting, the thickness of the sprayed coating 12 may be 200 to 3000 μm, for example.

Preferably, the formation of the sprayed coating 12 is followed by heat treatment for diffusion purposes. This achieves higher adhesion between the plug body 11 and sprayed coating 12. For the heat treatment for diffusion purposes, the plug is preferably held at 600 to 1250° C. for 10 minutes or longer, for example. The heat-treatment temperature is more preferably 600 to 1100° C.

The piercer plug 10 according to an embodiment of the present invention has been described. In the present embodiment, the XRF-Cr concentration in the sprayed coating 12 is 3 to 20 mass %. This will increase the wear resistance of the piercer plug 10.

The above-described embodiment illustrates an implementation where the plug body 11 is projectile-shaped. However, the plug body 11 may have any shape. For example, the piercer plug may be a plug body 13 with a protruding tip, as shown in FIG. 4, with a sprayed coating 12 formed thereon, or may be a plug body 14 of a divided construction, as shown in FIG. 5, with a sprayed coating 12 formed thereon.

The above-described embodiments illustrate implementations where the sprayed coating 12 is formed by arc spraying. However, a method for forming the sprayed coating 12 is not limited thereto. The sprayed coating 12 may be formed by, for example, plasma spraying, flame spraying, high-speed flame spraying, etc.

EXAMPLES

The present invention will now be described more specifically by means of examples. The present invention is not limited to these examples.

A model plug mainly composed of 0.15 C, 0.5 Si, 1.0 Ni, 0.5 Mn, 1.5 Mo, 3.0 W, and balance Fe was prepared and a sprayed coating was formed thereon. A positive-electrode wire and a negative-electrode wire were prepared by combining wires of a low-carbon steel, SUS 410 and SUS 430, and cored wires with different Cr concentrations to adjust the compound of the sprayed coating to be formed.

The XRF-Cr concentration in the sprayed coating was analyzed by the method described in connection with the embodiment. The X-ray fluorescence analyzer used was DP-2000 Delta Premium from JEOL Ltd., and the analysis was performed using ALLOY PLUS alloy analysis software from JEOL Ltd.

The Vickers hardness of the sprayed coating of each plug was measured. The Vickers hardness of each plug was obtained by conducting three-point measurement and determining the average of the measurements.

Table 1 shows the relationship between XRF-Cr concentration and average hardness. In Table 1, “-” in the column for XRF-Cr concentration indicates that the XRF-Cr concentration was below the lower limit for analysis.

TABLE 1 XRF—Cr concentration Vickers hardness Mark (mass %) (Hv) A — 320 B 1.04 345 C 3.03 380 D 7.85 400 E 11.52 450 F 15.53 500 G 18.11 520 H 22.30 550

As shown in Table, 1 the higher the XRF-Cr concentration, the higher the average Vickers hardness.

FIG. 6 shows a cross-sectional microphotograph of the sprayed coating labeled Mark A in Table 1. FIG. 7 shows a cross-sectional microphotograph of the sprayed coating labeled Mark C in Table 1. As illustrated in FIG. 7, a sprayed coating containing Cr was composed of metal and oxides, similar to the sprayed coating containing no Cr (FIG. 6). In these figures, relatively bright portions represent portions made from metal, while dark-gray portions represent portions made from oxides. The ratio between metal and oxides was substantially the same for each of the sprayed coatings prepared for the present application, where the proportion of oxides was about 44 to 55 vol %.

Subsequently, these plugs were used to conduct piercing tests, where the material to be processed was SUS 304, to measure the amount of wear of the coating. Table 2 shows the relationship between XRF-Cr concentration and amount of wear. The column labeled “Ratio of wear to conventional” in Table 2 lists the relative amounts of wear for the sprayed coatings of the plugs, where the amount of wear for the sprayed coating of the plug labeled Mark A represents the value 1.

TABLE 2 XRF—Cr Ratio of concentration wear to Mark (mass %) conventional Notes Remarks A — 1 (reference) — comp. ex. B 1.04 0.95 — comp. ex. C 3.03 0.70 — inv. ex. D 7.85 0.70 — inv. ex. E 11.52 0.60 — inv. ex. F 15.53 0.40 — inv. ex. G 18.11 0.50 — inv. ex. H 22.30 0.11 low piercing efficiency comp. ex.

As shown in Table 2, the higher the XRF-Cr concentration, the smaller the amount of wear. Particularly, an XRF-Cr concentration of 3 mass % or higher resulted in a reduced amount of wear of up to about 70% of the level for Mark A. On the other hand, an XRF-Cr concentration exceeding 20 mass % resulted in a reduction in piercing efficiency, making rolling difficult.

These results prove that an XRF-Cr concentration in the range of 3 to 20 mass % increases the wear resistance of a piercer plug.

Although embodiments of the present invention have been described, the above-described embodiments are exemplary only, intended to allow the present invention to be carried out. Accordingly, the present invention is not limited to the above-described embodiments, and the above-described embodiments, when carried out, may be modified as appropriate without departing from the spirit of the invention. 

1. A piercer plug, comprising: a plug body; and a sprayed coating formed on a surface of the plug body, the sprayed coating containing an iron-based alloy and an oxide of the iron-based alloy, a chromium concentration determined by analyzing the sprayed coating with X-ray fluorescence analysis being 3 to 20 mass %. 